Bisphenols, such as diphenylol propanes can be made by initially effecting reaction between a phenol and an alkyl ketone, such as acetone, in the presence of a mineral acid, for example, hydrogen chloride, as shown by "Purification Key Step in Making Bisphenol-A", European Chemical News, pages 38-40, July 16, 1965. There is also shown a subsequent acid removal step and a phenol removal step. In the course of isolating the diphenylol propane, a distillation step is required to effect the separation of volatiles. The final bisphenol is recovered by crystallization. Experience has shown that in the course of manufacturing or isolating bisphenols in the molten state or under distillation conditions, breakdown of the bisphenol often occurs resulting in the production of phenol and other decomposition by-products.
As taught in British Pat. No. 890,432, directed to Thermo-Stabilized Dihydroxydiarylalkanes and Cyclo-alkanes, various inorganic or organic compounds, such as secondary or tertiary alkaline earth phosphates, stannous oxalate, stannous oxide, tin dioxide, terephthalic acid, isophthalic acid, oxalic acid, etc., boron trioxide, antimony trioxide, etc., can be used as complexing agents to react with impurities to minimize the effect of such impurities on the decomposition of bisphenols under melt or distillation conditions. It is specifically taught that phthalic acid, due to its tendency to pass over to the anhydride at elevated temperatures, is not effective as a bisphenol stabilizing agent. Japanese patent SHO-45-22539 teaches that an aliphatic dicarboxylic acid ester can be added to a bisphenol-A distillation mixture to depress the decomposition and undesirable coloring of bisphenol-A. In addition, Japanese patent SHO-45-47854 teaches that high purity diphenylol propane can be obtained by heating the crude reaction product with materials such as polypropylene glycols, epoxy resins and epoxidized soybean oils.